1. Field of the Invention
This invention relates to a communication system and a method, for detecting a power exposure of a communication subscriber line to a commercial AC (alternating current) electrical power line.
2. Description of the Related Arts
There has sometimes taken place a trouble called a power exposure, that is, a commercial AC power line of 100 or 200 volts touches a communication line, for example, a telephone subscriber line, during a construction work of the lines, causing an excessive voltage in the telephone line resulting in a damage or fire of a telephone set in a home or a remote terminal RT. In order to protect the set or the remote terminal RT from the damage, a surge-absorber, typically consisting of a varistor and a posistor, has been provided between the ground and each wire of the subscriber line. The surge-absorber is effective for some degree to protect the apparatus; however, the surge absorber must be replaced with a new one when damaged by absorbing the excessive voltage caused from the power exposure, and does not alarm the damage. In order to detect the power exposure, the remote terminal RT may have been provided with a subscriber line tester. Subscriber line tester is connected to the subscriber line periodically or upon a request from the subscriber to test the subscriber line by being initiated from a control desk provided in a central office terminal COT located at the telephone exchanger side of the transmission line, opposite from the remote terminal RT. Therefore, in these methods there is a problem in that the power exposure cannot be instantly detected. In order to instantly detect the power exposure, the remote terminal may have been provided with a power exposure detector. In this method there is a problem in that the power exposure detector, such as a 60 Hz filter which is expensive, must be provided to each wire of every subscriber lines, resulting in an increase of the space of the required component, accordingly in the appratus cost.